1. Field of the Invention
The field of this invention relates to a radio frequency transmitter, an integrated circuit device, a wireless communication unit and a method therefor. The invention is applicable to, but not limited to, a method and apparatus for generating a radio frequency signal for transmission over a radio frequency (RF) interface.
2. Description of the Prior Art
Advances in the deep sub-micron CMOS (Complimentary Metal-Oxide Semiconductor) process have lead to digital circuits becoming smaller and more power efficient. However, analog circuits do not scale well with the deep sub-micron CMOS process. It is therefore desirable for devices, such as radio frequency (RF) transmitters, to remove as many analog components as possible with the assistance of digital signal processing algorithms.
Furthermore, conventional RF transmitters use linear power amplifiers. Accordingly, the power efficiency of such conventional RF transmitters is usually very low due to the low efficiency of the linear PAs used therein. Switch-mode PAs have very high efficiency in comparison, which make such switch-mode PAs an attractive alternative to conventional linear PAs within RF transmitters.
Thus, an RF transmitter that can utilize switch-mode PAs through the assistance of digital processing algorithms to reduce the size and improve power efficiency is highly desirable. However, switch-mode PAs normally exhibit a highly non-linear input-output relationship. Furthermore, to meet stringent co-existence requirements of various wireless standards, noise shaping techniques are required.
Digital polar transmitters are a type of known transmitter design that utilizes switch-mode PAs whilst also taking advantage of CMOS process technology. Accordingly, such digital polar transmitters are able to achieve high power efficiency, whilst requiring only a small area. However, a problem with these known transmitter designs is that, due to the inherent bandwidth expansion characteristics of the AM (amplitude modulation) and PM (phase modulation) signals in a polar architecture, they are only suitable for narrowband modulated signals.
Hybrid polar transmitter designs take advantage of two dimensional (in-phase/quadrature) modulation to enable wideband phase modulation to be achieved. However, a problem with such hybrid polar transmitters is that they suffer from both amplitude and phase quantization noise, thus requiring significant noise shaping.
In-phase/Quadrature (IQ) RF digital to analogue convert (DAC) based transmitters are also known, which combine the functionalities of a DAC and a mixer, with the output of the I/Q RF DAC being combined in the analogue (RF) domain. However, such transmitter designs require a linear PA, and direct I/Q RF digital to analogue conversion is less power efficient than a digital polar transmitter design.
Another known RF transmitter design utilizes adaptive pre-distortion within a feedback loop using a delta-sigma modulator for automatic inversion of power amplifier non-linearity. Such a design is simple and allows for the use of low-precision DACs. However, this design still comprises a generally conventional architecture, and so PA efficiency is low.
It is expected that digitally assisted/intensive RF transmitters will become increasingly desirable. However, the digital signal processing in an RF transmitter front-end is different than the digital signal processing in baseband applications since it is related to the RF frequency, and thus is usually required to run at high speeds and with small energy consumption, whilst also being limited to small circuit size. However digital algorithms are limited by the availability of circuit speed; therefore finding simple yet efficient and effective digital algorithms is crucial from the implementation point of view. In published literature currently available there are discussions on digital algorithms that operate at very high clock frequencies, such as four times the carrier frequency. However, such clock frequencies are in practice not implementable.
Thus, a need exists for an improved RF transmitter, and method of operation therefor.